Because Shaker doctrine forbids sexual reproduction among members, the movement could never sustain its population the way an ordinary religious community does, through children raised inside it; every Shaker had to be either a convert who joined as an adult or a child taken in through the community's practice of accepting orphans and indentured children, some of whom later chose to stay and formally join as adults while others left at maturity. Stephen Stein's history of the movement traces how this structural dependence on continuous conversion left Shaker numbers acutely sensitive to the broader currents of American religious enthusiasm: membership peaked at several thousand across nineteen communities in the mid nineteenth century, during a period of active revivalism, then declined steadily as the revivalist climate that had fed conversions cooled and as an increasingly secular and mobile American society offered fewer candidates drawn to a life of celibate communal labor. By the twentieth century the same doctrine that had given the movement its distinct communal economy, since celibate members held no private households or heirs and devoted their labor entirely to the shared community, left it unable to replace elderly members as fast as it lost them, so a doctrine understood by Shakers as living already within the resurrection became, by ordinary demographic mechanics, a direct cause of the movement's near disappearance.